I. Field of the Invention
The present invention relates to a variable compression ratio (VCR) piston assembly and, more particularly, to such an assembly for a two stroke internal combustion engine.
II. Description of the Prior Art
The previously known VCR piston assemblies typically comprise an inner piston member connected in the usual manner to a connecting rod and an outer piston member carried by and axially movable relative to the inner member. Relative movement of these members varies the compression ratio of the particular cylinder in which the piston moves. Clearance spaces are provided between the top and bottom ends of the inner and outer members and these form upper and lower chambers which vary conversely in volume in relation to the relative movement of the piston members. An incompressible fluid, such as the engine lubrication oil, is supplied to these chambers in a manner which automatically regulates movement of the members to gradually change the compression ratio until a predetermined combustion pressure has been achieved. The hydraulic system then varies the movement of the members in a manner which tends to maintain a uniform maximum combustion chamber pressure.
These previously known VCR piston assemblies have been designed for four cycle engine operation. With a four cycle engine, due to inertia forces the outer member of the piston is caused to move away from the inner member at the upper end of the exhaust stroke and at the early part of the downward intake stroke. As the outer member moves relative to the inner piston member, the upper chamber of the piston assembly fills with oil in preparation for the subsequent combustion stroke of the engine. During the combustion stroke of the VCR piston assembly, the oil is expelled from the upper chamber past a pressure responsive release valve and into the oil collection system for the engine.
While these previously known VCR piston assemblies have proven adequate in operation for four stroke engines, such VCR piston assemblies are inoperable with two cycle engines. With a two cycle engine, the exhaust stroke present in a four cycle engine is omitted so that the outer piston member is unable to adequately axially move away from the inner piston member in preparation for the combustion stroke of the engine.